Some types of filters are generally fabricated by coating a fibrous substrate with a liquid suspension of particles. The coated substrate is dried and subsequently heated to cause the particles to adhere to the substrate.
U.S. Pat. No. 4,025,666 issued on May 24, 1977 to Pierce, hereinafter referred to as Pierce, the entire disclosure of which is incorporated herein by reference, teaches a method of decorating the border panel or any other area of a fibrous cardboard picture mat. A finely divided pigmented material is brushed into the areas to be colored. The paper surface holds only a limited amount of the dry powder, and excess powder is removed.